1、Additional Electrical CharacteristicsThreshold Adjustment Ion ImplantationVoltage breakdown mechanisms Several factors, such as fixed oxide charge, M-S work function difference, oxide thickness, and semiconductor doping, influence VT. Although all of these parameters may be fixed in a particular des
2、ign and fabrication process, the resulting VTmay not be acceptable for all applications. Ion implantation can be used to change and adjust the substrate doping near the oxidesemiconductor surface to provide the desired VT. Ion implantation is used for more than doping the channel. It is used extensi
3、vely as a standard part of device fabrication; for example, it is used to form the source and drain regions of the transistorThreshold Adjustment Ion Implantation To change the doping and thereby change the VT, a precise, controlled number of either donor or acceptor ions are implanted into the semi
4、conductor near the oxide surface. When an MOS device is biased in either depletion or inversion and when the implanted dopant atoms are within the induced space charge region, then the ionized dopant charge adds to the maximum space charge density, which controls the VTMOS. The effect of doping on t
5、he VT:An implant of acceptor ions into either a p- or n-type substrate will shift the threshold voltage to more positive values, while an implant of donor ions will shift the threshold voltage to more negative values.Threshold Adjustment Ion Implantation Assume that DIacceptor atoms per cm2are impla
6、nted into a p-type substrate directly adjacent to the oxidesemiconductor interface. The shift in VTdue to the implant is If donor atoms were implanted into the p-type substrate, the space charge density would be reduced; thus, the VTwould shift in the negative voltage direction.According to the dist
7、ribution of implanted ions, two cases are discussed:The first approximation ：Delta distributionIon-implanted profile approximated by a delta function(12.39)Threshold Adjustment Ion Implantation If the induced space charge width is greater than xIat the threshold inversion point, then a new expressio
8、n for xdTmust be derived. We can apply Poissons equation and show that the maximum induced space charge width after the step implant isIon-implanted profile approximated by a step functionThe second type : the step junction If the induced space charge width xdTat the threshold inversion point is les
9、s than xi, then the threshold voltage is determined on the basis of a semiconductor with a uniform doping concentration of Ns.where the subscript 0 indicates the preimplant values. The threshold voltage after a step implant for the case when xdT xIcan be written as where VT0is the preimplant thresho
10、ld voltage. The parameter DIis given by ：DI=(Ns-Na)xI which is the number per cm2of implanted ions. The preimplant threshold voltage is ：(12.41)(12.42)(12.43)The second type : the step junction The actual implant dose versus distance is neither a delta function nor a step function; it tends to be a
11、Gaussian-type distribution. The threshold shift due to a nonuniform ion implant density may be defined as the shift in curves of Ninv versus VG, where Ninvis the inversion carrier density per cm2. This shift corresponds to an experimental shift of drain current versus VGwhen the transistor is biased
12、 in the linear mode. The criteria of the threshold inversion point as S2 fpin the implanted devices have an uncertain meaning because of the nonuniform doping in the substrate. The determination of the threshold voltage becomes more complicated and will not be made here.The second type : the step junction
